Such machines are referred to as band sealers or chain band sealers, and have utilized opposing endless chains for gripping and physically moving the bag tops with the bands. The bags are most generally additionally supported by an underlying belt conveyor traveling at the same speed as the bag-gripping chains and the bands through which heating and cooling of the bags is effected.
Such prior band sealers have been in commercial use for a number of years and have used a pair of endless bands arranged with film-engaging runs above the conveyor chains and confronting each other in closely spaced relation to receive and carry the film laminae therebetween. In such machines, each band has moved successively in the film-engaging run past a heating station and then past a cooling station; and subsequently the band progresses along a return run to the receiving end of the machine at which the band again repeats the cycle.
In such prior machines, each of the two bands is subjected to about 450.degree. F. at the heating station and immediately subjected to cooling temperatures which are less than room temperatures and may be about tap water temperature of 40.degree. - 50.degree. F. These rapid and repeated changes of temperature cause severe internal stresses in the bands, and often cause warping and deformation in the form of wrinkling into a corrugated or "washboard" shape, and oftentimes the bands deform by bowing in a transverse direction. This warping is all the more acute because oftentimes the band is only heated along a narrow zone between the side edges of the band. Of course, such warping and deformation makes the band unusable for producing high grade seals in such film. In normal operation, such sealing bands are necessarily replaced several times a week, and in processing certain types of bag materials, the bands must oftentimes be replaced several times during each working day.
The repeated extreme heating and extreme cooling of the bands in each cycle of operation produces such a degree of warping and wrinkling of the bands that the band surfaces which are supposed to lie flush against the film being sealed, will instead in some instances bow away from the bag so that practically no heat is transferred to it, or, in other instances, a corrugated band will only engage the film material at intermittent or spaced locations so that continuous seals cannot be produced.
Such band deformation becomes extremely significant at the cooling section of such prior machines. The cooling bars must apply very significant pressure to partially flatten the band against the seal area so that heat may be extracted, and as a result production of a shrink seal in the bag is impossible. In a shrink seal, the seal zone grows in thickness as compared to the combined laminae thicknesses adjacent the seal zone, and the seal is very strong. However, because such prior machines must apply such significant pressure to the band and seal area, growth of the thickness of the seal has been impossible, and the contrary result, reduction in thickness, has oftentimes resulted.
The repeated substantial changes between such temperature extremes in each cycle of operation requires the heating bars at the heating station to apply temperatures of 400.degree. - 450.degree. F. to the band so as to be assured of adequate reheating of the band from its cooled condition and heating of the film to high enough temperatures to melt the film for welding or fusing to produce a seal. In exposing the film to such high temperatures, the surface of the film oftentimes becomes deformed, and the seal area of the film becomes excessively soft and weak. When the machine is run at speeds to produce reasonable output, such as up to 600 inches per minute, the cooling section cannot adequately cool the seals, and, accordingly, the film or bag must be carefully handled for a time after it emerges from the sealing machine. Such bags cannot be immediately rough handled.
Of course, this need for careful handling the sealed bags requires more room in the plant or processing area. In such prior machines, the bands continuously clamp the film as the film passes first along the heating station and then along the cooling station. As the film laminae are sealed, the seal area becomes thinner than the adjacent unsealed areas. For instance, if two 4 mil thick film laminae (totaling 8 mils in thickness) are being sealed together, the thickness at the seal zone or strip may be only approximately 6 mils in thickness. Accordingly, there is a shear line in each laminae at the edge of the seal zone where the laminae abruptly changes in thickness. The film is likely to be weakened along such a shear line, resulting in a weak seal.
In prior band sealers, coatings on the steel bands have been required to assure that the seal is released from the band at the end of the first heating-cooling run. Often the band is coated with a thin slippery plastic coating or laminae affixed to the steel band and made of such commercial product known by its trademark TEFLON. Other release agents have also been used, such as light oil, the coating of the steel bands with TEFLON or other similar materials makes the bands extremely expensive and difficult to obtain in remote locations. When the coating is worn off, the band becomes essentially useless. Such bands in prior machines have necessarily been extremely thin, approximately 0.005 of an inch thick in order to adequately perform the purpose intended. This extreme thinness has contributed materially to the substantial expense of these bands, and has contributed to the delicateness of the bands producing a propensity to readily break under the stresses created in the prior machines.
In such prior machines, the heater bars for supplying heat to the bands have been mounted to be stationary with respect to the frame of the machine, and, in such instances, the heater bars must be located so that they will be close to the traveling bands, but allowance must be made for the thickness of the film material as well as some additional spacing so that the film material will not produce jamming of the machine in the event that wrinkles or multiple thicknesses of film material are encountered.
In other instances, the heater bars are mounted on springs relative to the machine so that the entire heater bar is movable inwardly and outwardly for applying pressure against the moving band and the film material being heat sealed so that the sealing is effected under compressive pressure and further, some allowance is made for the heater bars to separate from each other and allow the bands to separate from each other when wrinkled portions of the film or multiple thicknesses of the film pass through the machine during sealing. Under these circumstances, there is essentially no uniformity in the amount of pressure which is being applied to the laminae being sealed. The pressure being applied is either too little or too much. This problem of pressures becomes extremely acute because of the warping or wrinkling of the metal bands after they have been used for a short period of time. The metal bands, by reason of the wrinkling or transverse bowing, occupy space which is intended to accommodate wrinkles in the film, and the heating bars must be relocated or adjusted from time to time.
Additionally, in such prior machines, the life of the very thin bands that have been required has been adversely affected by the obstacles which are encountered along the heating and cooling runs of the bands. The band mounting wheels are allowed to remain sufficiently apart so that multiple thicknesses of the bag material can be passed between the wheels, but, in most cases, the heater bars and then the cooling bars along the runs of the bands will guide the metal bands into more closely spaced relation with each other. At the particular location wherein the metal bands first encounter the heater bars, the metal band is suddenly guided around a corner of small radius, thereby bending the band. This same occurrence happens at the downstream end of the heating bar, and then again at the leading end of the cooling bar, and again at the trailing end of the cooling bar. This rather constant bending of the band as it travels along the film sealing run continuously works the band so as to adversely affect the band life. When this working is compounded with the continual warping or bowing, the shortening of the band life is compounded.